One of the most important families of propylene polymers comprises heterophasic copolymer compositions made of a high crystallinity propylene polymer fraction and a low crystallinity elastomeric component (for instance, a propylene-ethylene copolymer).
These compositions may be prepared by mechanical blending of the two main polymeric components, and may further be prepared via the sequential polymerization technique where the relatively high crystalline propylene polymer (sometimes called the crystalline matrix) is prepared in a first polymerization reactor and then transferred to a second polymerization reactor, where the low crystallinity elastomeric component is formed.
In this type of process, the porosity of the relatively high crystallinity polymer matrix may affect the incorporation of the elastomeric fraction into the crystalline matrix.
As a general rule, the higher the porosity of the polymer matrix produced in the first step, the higher is the amount of the resulting elastomeric component that can be incorporated within the matrix in the second polymerization step.
If the porosity of the matrix is poor, the presence of an excessive amount of elastomeric polymer fraction on the particles surface may considerably increase the tackiness of the particles, which gives raise to agglomeration phenomena causing reactor problems such as reactor wall sheeting, plugging and clogging.
A macroscopic measurement of the polymer porosity may be obtained by the polymer bulk density. The bulk density or apparent density is the mass per unit of volume of a material, including voids inherent in the material. In the case of polymer particles of a regular morphology, relatively low values of bulk density indicate a relatively high porosity of the polymer powder. It would therefore be useful to produce a propylene polymer endowed with both higher porosity (lower bulk density) and high crystallinity in the first polymerization step.
One option for producing crystalline polymers with a defined level of porosity is to polymerize propylene with a catalyst having a defined porosity.
As disclosed in EP Pat. Doc. 395083, a catalyst having defined porosity can be obtained using adducts of the general formula MgCl2.mEtOH.nH2O, where m is between 1 and 6 and n is between 0.01 and 0.6, from which a certain amount of alcohol is removed, thereby creating a porous precursor which is then converted into a catalyst component by reaction with a titanium compound containing at least one Ti—Cl bond.
As a drawback, an increase in catalyst porosity may lead to a corresponding decrease in catalyst polymerization activity.
The present disclosure describes new MgCl2.mEtOH.nH2O adducts having specific chemical and physical properties useful as precursors for catalyst components able to generate polymers comprising lower bulk density without exhibiting a reduction in polymerization activity.